| 1: | PC1 sits on an Ethernet and uses IP address 130.1.1.1. PC1 needs to send a packet to an FTP server whose IP address is 19.1.1.1. To what IP address should PC1 forward the IP packet next? |
| A1: | Answer: PC1 should send the packet to its default gateway IP address. |
| 2: | Imagine that PC1, with IP address 130.1.1.1, needs to send a packet. PC1 decides to send the packet to its default router, at IP address 130.1.1.2. PC1 has never sent a packet to that router. Two messages must flow over the network before PC1 can forward the packet. Identify the acronym for the protocol that defines those two messages, and state what the letters in the acronym stand for. |
| A2: | Answer: ARP, which stands for Address Resolution Protocol, defines a means for a LAN device to discover the LAN MAC address of another IP host on the same LAN. |
| 3: | Imagine that PC1, with IP address 130.1.1.1, needs to send a packet to its default router at IP address 130.1.1.2. Both PC1 and the router are attached to the same Ethernet switch, along with PCs Larry, Moe, and Curly. PC1 doesn't know the router's Ethernet MAC address. Name the message that PC1 sends to try to find out the router's MAC address, and identify which of the devices on the LAN will receive the message. |
| A3: | Answer: PC1 sends an ARP broadcast asking for 130.1.1.2 to reply with its MAC address. The ARP broadcast has a broadcast destination LAN MAC address. So, the switch forwards the ARP broadcast to the router, as well as to Larry, Moe, and Curly. |
| 4: | PC1, with IP address 130.1.1.1, needs to send a packet to Curly, at IP address 130.1.1.3. PC1's default router is the router whose IP address is 130.1.1.2. Both PCs and the router are attached to a single Ethernet switch and are in the same subnet. Describe the role of the default router for routing this packet from PC1 to Curly. |
| A4: | Answer: PC1 first notices that the destination, 130.1.1.3, is in the same subnet as its own IP address (130.1.1.1). As a result, PC1 knows that it can send the packet directly to Curly. PC1 sends the packet directly to Curly, ignoring its default router. |
| 5: | PC1 (130.1.1.1) is attached to an Ethernet switch. It needs to send a packet to a web server (19.1.1.1). The IP packet is sitting in memory in PC1. Describe the process of encapsulation that is required before PC1 can transmit the bits. |
| A5: | Answer: Only an Ethernet frame can be transmitted onto an Ethernet LAN. Because the data link TCP/IP layer sits just below the internetwork layer, a host can reasonably expect that the Internetwork layer IP packet can be encapsulated inside a data link layer framein this case, an Ethernet frame. PC1 adds an Ethernet header and trailer to the IP packet, creating an Ethernet frame. The frame can then be transmitted over the Ethernet. |
| 6: | Refer to Figure 11-15 for this question. In the sample network, PC1 is sending a packet to PC2. Describe whose IP and MAC addresses would be contained in the Ethernet frame that passes between PC1 and R1. Figure 11-15. Internetwork Referenced in Some of the Questions in Chapter 11 
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| A6: | Answer: It would contain a source IP address of 150.1.1.1, a destination IP address of 150.1.3.3, a source MAC address of PC1's MAC address, and a destination MAC address of R1's E1 MAC address. |
| 7: | Refer to Figure 11-15 for this question. In the sample network, PC1 is sending a packet to PC2. Describe whose IP and MAC addresses would be contained in the Ethernet frame that passes between R2 and PC2. |
| A7: | Answer: It would contain a source IP address of 150.1.1.1, a destination IP address of 150.1.3.3, a source MAC address of R2's E2 MAC address, and a destination MAC address of PC2's MAC address. |
| 8: | Refer to Figure 11-15 for this question. In the sample network, PC1 is sending a packet to PC2. Describe the contents of the routing table entry on R1 that would be needed for R1 to forward the packet correctly. |
| A8: | Answer: The routing table would have to list the destination subnetin this case, 150.1.3.0. In that same routing table entry, the forwarding instructions would refer to the outgoing interfaceE2 in this caseas well as the next-hop router, which is 150.1.2.2 in this case. |
| 9: | Refer to Figure 11-15 for this question. In the sample network, PC1 is sending a packet to PC2. Describe the contents of the routing table entry on R2 that would be needed for R2 to forward the packet correctly. |
| A9: | Answer: The routing table would have to list the destination subnetin this case, 150.1.3.0. In that same routing table entry, the forwarding instructions would refer to the outgoing interfaceEthernet2 in this casebut no next-hop router because R2 would not need to forward the packet to another router. |
| 10: | Define the term "routing." |
| A10: | Answer: Routing is the process of receiving a packet and comparing the IP destination address to a list called a routing table, which contains possible destinations. The matched entry defines where to forward the packet next so that it eventually reaches the destination. |
| 11: | Define the term "default gateway." |
| A11: | Answer: For a host computer, the default gateway is the IP address to which that host can forward packets, relying on that router (gateway) to know how to forward the packet to the destination. |
| 12: | Describe the differences between routing when you are not using subnetting and when you are using subnetting. |
| A12: | Answer: The routing process is no different whether you use subnetting or not. The routing table lists the groups of IP addresses that can be reached, whether they are IP networks or IP subnets. |
| 13: | Describe the logic that a host computer uses when deciding whether it needs to send a packet to its default gateway or directly to the destination host. |
| A13: | Answer: The sending host looks at the destination IP address of the packet and compares it to the network or subnet to which the sending host is connected. If the destination of the IP packet is on the same network/subnet as the sender, the sender sends the packet directly to that host; otherwise, it sends the packet to the default gateway. |
| 14: | Referring to Figure 11-15, between the time that PC1 sends the packet and PC2 receives it, many things might have changed. Explain what is the same and what is different about the transmitted data. Compare what PC1 sends versus what PC2 receives. |
| A14: | Answer: PC1 creates an Ethernet header and trailer to put around the IP packet, but R1 strips off the Ethernet header and trailer. R1 creates a new Ethernet header and trailer, but R2 strips those off. The only thing that is delivered across the whole internetwork from PC1 to PC2 is the IP packet. |
| 15: | This chapter suggests that you will find three items in a single routing table entry. What are they, and which one(s) are useful for knowing where to send the packet next? |
| A15: | Answer: Although there might be more information in a single routing table entry in real life, this chapter focused on the destination network or subnet, the outgoing interface, and the next-hop router. The last two of these tell the router how to forward a packet. |
| 16: | Of the three parts of a routing table entry from the previous question, which one of these parts might not always be needed? Using R2 from Figure 11-15 as a reference, describe one routing table entry where that field would not be needed in the routing table entry. |
| A16: | Answer: The next-hop router field is not always needed. For instance, R2 does not need to send the packets to another router to forward packets to subnet 150.1.3.0 because it is directly connected to that subnet. R2's routing table entry for subnet 150.1.3.0 would not have a next-hop router field. |
